1. Field of the Invention
The present invention relates to a light receiving optical system to be incorporated in a spectral device such as a spectrophotometer, and more particularly to a light receiving optical system operable to switch a view angle, and a spectrophotometer incorporated with the light receiving optical system.
2. Description of the Related Art
FIG. 7 is a diagram showing an example of a conventional light receiving optical system to be incorporated in a spectral device such as a spectrophotometer. The light receiving optical system 900 shown in FIG. 7 has an aperture plate (hereinafter, called as a “view angle switching aperture plate”) 902 for view angle switching control. The view angle switching aperture plate 902 is formed with an aperture 902a having a radius Ra for providing a view angle of 1°, and an aperture 902b having a radius Rb for providing a view angle of 0.1° with respect to an image plane of the objective lens 901. Immediately after a light flux 903a incident onto the objective lens 901 is converged i.e. condensed while passing through the aperature 902a or 902b, the light flux 903a is incident onto a conversion fiber 904 through an incident end 904a. 
The view angle θ is given by: 2*a tan(Ra/F1) or 2*a tan(Rb/F1), where F1 is a distance between the objective lens 901 and the view angle switching aperture plate 902, Ra is a radius of the aperture 902a, Rb is a radius of the aperture 902b, and the symbol “*” indicates multiplication. The conversion fiber 904 has the incident end 904a of a circular shape in cross section at one end thereof, and a slit-shaped exit end 904b at the other end thereof, (see FIG. 4 to be described later). The exit end 904b serves as an incident slit of a polychromator 905. In this arrangement, a light flux incident through the circular incident end 904a is allowed to be efficiently incident onto a diffraction optical system (not shown) in the polychromator 905 with a predetermined slit width.
The view angle θ is switched by moving e.g. sliding the view angle switching aperture plate 902 to such a position that one of the apertures 902a and 902b is aligned in front of the incident end 904a. The incident site i.e. the incident position of incident light (i.e. the light flux 903a converged by the objective lens 901) onto the incident end 904a may be displaced due to a reproduction error with respect to the aperture position in the switching operation. This may displace the exit position of exit light 903c through the exit end 904b, thereby impairing measurement reproducibility.
On the other hand, an aperture 906a of an aperture stop 906 disposed near the objective lens 901 is adapted to regulate the incident light flux i.e. the incident light amount with respect to the aperture 902a or the aperture 902b of the view angle switching aperture plate 902 to define a possible incident angle of incident light. The possible incident angle is substantially maintained after propagation of the light flux through the conversion fiber 904, and is substantially equal to an exit angle i.e. a divergent angle ω of the exit light 903c. Preferably, the exit angle ω is substantially coincident with a possible incident angle with respect to a concave diffraction grating (not shown) in order to sufficiently utilize the performance of the concave diffraction grating in the diffraction optical system of the polychromator 905. The aperture diameter of the aperture stop 906 is defined to realize the above arrangement. The effective brightness of the whole optical system is substantially determined based on the effective area of the exit aperture i.e. the exit end 904b, in other words, based on the radius Ra of the aperture 902a, the radius Rb of the aperture 902b, and the exit angle ω. In the case where the aperture 902b for providing the view angle of 0.1° is aligned in front of the incident end 904a of the polychromator 905, the amount of light to be incident onto the polychromator 905 is merely one-hundredth of the amount of light to be incident onto the polychromator 905 in the case where the aperture 902a for providing the view angle of 1° is aligned in front of the incident end 904a of the polychromator 905, unconditionally i.e. irrespective of the performance of the objective lens 901.
In a light receiving optical system 920 disclosed in e.g. Japanese Unexamined Patent Publication No. 2001-264166 (D1), as shown in FIG. 8, a relay lens 923 is arranged between a view angle switching aperture plate 921 and an incident end 922a of a conversion fiber 922. The relay lens 923 is arranged at such a position that a focal point thereof is aligned with the center of an aperture 921a or an aperture 921b of the view angle switching aperture plate 921 to form an image 922c through the incident end 922a of the conversion fiber 922 immediately behind an objective lens 924. The image 922c serves as an aperture stop image for regulating a light flux to be incident onto a polychromator 925. Similarly to the arrangement example shown in FIG. 7, the conversion fiber 922 has a slit-shaped exit end 922b serving as an incident slit of the polychromator 925. Also, similarly to the arrangement example shown in FIG. 7, switching of the view angle .theta. is performed by moving the aperture plate 921 to such a position that one of the aperture 921a and the aperture 921b is aligned in front of the incident end 922a. In the example shown in FIG. 8, however, there is no likelihood that the incident site of a light flux 926a onto the incident end 922a may be displaced resulting from a position error of the aperture 921a or the aperture 921b to be used in the switching operation, because the light flux 926a passing through the aperture 921a or the aperture 921b is incident onto the entirety of the incident end 922a, irrespective of the aperture position of the aperture 921a and the aperture 921b. 
However, since the angle distribution of incident light i.e. the light flux 926a with respect to the incident end 922a is changed resulting from the position error, transmission efficiency of the conversion fiber 922 is changed accordingly. Also, the angle distribution of exit light 926c is changed resulting from a change in angle distribution of the incident light. As a result, the diffraction efficiency i.e. the reflection efficiency of the concave diffraction grating is changed, which may impair measurement reproducibility. Assuming that the aperture 921a is operable to provide a view angle of 1.degree., and the aperture 921b is operable to provide a view angle of 0.1°, similarly to the arrangement example shown in FIG. 7, the amount of incident light to be incident onto the polychromator 925 in the case where the aperture 921b is aligned in front of the incident end 922b of the polychromator 925 is unconditionally one-hundredth of the amount of light to be incident onto the polychromator 925 in the case where the aperture 921a is aligned in front of the incident end 922b of the polychromator 925.